FIG. 1.

Adaptive acid tolerance and growth-phase-dependent acid tolerance in E. coli O157:H7 strain P1342. (A) Survival of exponential-phase cells challenged at pH 3.0. Cells were grown to mid-exponential phase in TPB at 30°C and either unadapted (open squares) or adapted at pH 5.0 for 1 h (solid squares) prior to challenge. One culture was adapted in the presence of chloramphenicol (5 μg ml⁻¹) added 15 min prior to adaptation (solid triangles). (B) Acid tolerance measured throughout growth. An overnight culture was used to inoculate TPB (inoculum, 0.5% [vol/vol]), and acid tolerance (percent survival after a 90-min challenge at pH 3.0) was measured at time intervals during growth (solid squares). Growth was recorded by measuring the OD₆₀₀ (open triangles). For panels A and B, the error bars represent the standard deviations from the means (n = 3).